Lithographic apparatus and device manufacturing method

ABSTRACT

A method and lithographic apparatus in which a surface of a sensor is protected from dissolution in a liquid through application of a bias voltage to the surface with respect to one or more parts which are also exposed to the liquid.

FIELD

The present invention relates to a lithographic apparatus and a methodfor manufacturing a device.

BACKGROUND

A lithographic apparatus is a machine that applies a desired patternonto a substrate, usually onto a target portion of the substrate. Alithographic apparatus can be used, for example, in the manufacture ofintegrated circuits (ICs). In that instance, a patterning device, whichis alternatively referred to as a mask or a reticle, may be used togenerate a circuit pattern to be formed on an individual layer of theIC. This pattern can be transferred onto a target portion (e.g.comprising part of, one, or several dies) on a substrate (e.g. a siliconwafer). Transfer of the pattern is typically via imaging onto a layer ofradiation-sensitive material (resist) provided on the substrate. Ingeneral, a single substrate will contain a network of adjacent targetportions that are successively patterned. Known lithographic apparatusinclude so-called steppers, in which each target portion is irradiatedby exposing an entire pattern onto the target portion at one time, andso-called scanners, in which each target portion is irradiated byscanning the pattern through a radiation beam in a given direction (the“scanning”-direction) while synchronously scanning the substrateparallel or anti-parallel to this direction. It is also possible totransfer the pattern from the patterning device to the substrate byimprinting the pattern onto the substrate

It has been proposed to immerse the substrate in the lithographicprojection apparatus in a liquid having a relatively high refractiveindex, e.g. water, so as to fill a space between the final element ofthe projection system and the substrate. The point of this is to enableimaging of smaller features since the exposure radiation will have ashorter wavelength in the liquid. (The effect of the liquid may also beregarded as increasing the effective NA of the system and alsoincreasing the depth of focus.) Other immersion liquids have beenproposed, including water with solid particles (e.g. quartz) suspendedtherein.

However, submersing the substrate or substrate and substrate table in abath of liquid (see, for example, U.S. Pat. No. 4,509,852, herebyincorporated in its entirety by reference) means that there is a largebody of liquid that must be accelerated during a scanning exposure. Thisrequires additional or more powerful motors and turbulence in the liquidmay lead to undesirable and unpredictable effects.

One of the solutions proposed is for a liquid supply system to provideliquid on only a localized area of the substrate and in between thefinal element of the projection system and the substrate (the substrategenerally has a larger surface area than the final element of theprojection system). One way which has been proposed to arrange for thisis disclosed in PCT patent application WO 99/49504, hereby incorporatedin its entirety by reference. As illustrated in FIGS. 2 and 3, liquid issupplied by at least one inlet IN onto the substrate, preferably alongthe direction of movement of the substrate relative to the finalelement, and is removed by at least one outlet OUT after having passedunder the projection system. That is, as the substrate is scannedbeneath the element in a −X direction, liquid is supplied at the +X sideof the element and taken up at the −X side. FIG. 2 shows the arrangementschematically in which liquid is supplied via inlet IN and is taken upon the other side of the element by outlet OUT which is connected to alow pressure source. In the illustration of FIG. 2 the liquid issupplied along the direction of movement of the substrate relative tothe final element, though this does not need to be the case. Variousorientations and numbers of in- and out-lets positioned around the finalelement are possible, one example is illustrated in FIG. 3 in which foursets of an inlet with an outlet on either side are provided in a regularpattern around the final element.

SUMMARY

A number of components in an immersion lithography apparatus may have asemi-conductive or conductive coating or be made of a semi-conductive orconductive material. For example, an optical sensor is commonly used inorder to provide a method for sensing alignment or a wave front. Such asensor typically employs a metallic or dielectric coating in the form ofa lithographic pattern in order to create spatially varying transmissionor reflection. This sensor is commonly exposed to liquid in an immersionlithography apparatus.

Under the influence of DUV light, the conductive or semi-conductivecoating or material may degrade, thus causing loss of sensor performanceover time. This usually involves periodic recalibration of the sensorand reduces sensor lifetime. The degradation may be due to thedissolution of the sensor coating or material in the immersion liquid.This degradation may possibly lead to particles of the sensor coating ormaterial breaking off, which may cause the further problem ofcontamination of the immersion liquid and of the substrate.

As an example, a sensor may employ a chrome coating since chrome caneasily be patterned to give the required pattern. However, chrome may besusceptible to degradation in an immersion liquid, possibly throughionization in the presence of DUV. Ionization produces chromium ionswhich may dissolve into the immersion liquid.

Accordingly, it would be advantageous, for example, to provide alithographic apparatus in which degradation of a sensor coating ormaterial is reduced or avoided, and in which the contamination of theimmersion liquid through dissolution of the sensor coating or materialis reduced or avoided.

According to an aspect of the invention, there is provided alithographic projection apparatus, comprising:

a substrate table constructed to hold a substrate;

a projection system configured to project a patterned radiation beamonto the substrate;

a liquid supply system configured to at least partially fill a spacebetween the projection system and the substrate table with a liquid;

a sensor having a surface, the surface configured to be at leastpartially exposed to the space; and

an electrical power supply configured to supply a bias voltage betweenthe surface of the sensor and a conductive or semi-conductive element,which element is configured to be at least partially exposed to thespace.

According to an aspect of the invention, there is provided alithographic apparatus, comprising:

an illumination system configured to condition a radiation beam;

a support constructed to hold a patterning device, the patterning deviceconfigured to impart the radiation beam with a pattern in itscross-section to form a patterned radiation beam;

a substrate table constructed to hold a substrate;

a projection system configured to project the patterned radiation beamonto a target portion of the substrate;

a liquid supply system configured to at least partially fill a spacebetween the projection system and the substrate table with a liquid;

a sensor having a semi-conductive or conductive surface configured to beat least partially exposed to the space; and

an electrical power supply configured to supply a bias voltage betweenthe surface of the sensor and an element configured to be at leastpartially exposed to the space.

According to an aspect of the invention, there is provided a devicemanufacturing method, comprising:

supplying a liquid to a space between a projection system of alithographic apparatus and a substrate;

applying a bias voltage between a surface of a sensor of thelithographic apparatus and a conductive or semi-conductive element whichelement is at least partially exposed to the liquid, the surface beingat least partially exposed to the liquid; and

projecting a patterned beam of radiation, using the projection system,through the liquid onto the substrate.

According to an aspect of the invention, there is provided a method ofreducing dissolution of a surface of a sensor of a lithographicapparatus, the method comprising:

supplying a liquid to a space between a projection system of alithographic apparatus and a substrate; and

applying a bias voltage between a surface of a sensor of thelithographic apparatus and a conductive or semi-conductive element whichelement is at least partially exposed to the liquid, the surface beingat least partially exposed to the liquid.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying schematic drawings in whichcorresponding reference symbols indicate corresponding parts, and inwhich:

FIG. 1 depicts a lithographic apparatus according to an embodiment ofthe invention;

FIGS. 2 and 3 depict a liquid supply system for use in a lithographicprojection apparatus;

FIG. 4 depicts another liquid supply system for use in a lithographicprojection apparatus; and

FIG. 5 depicts a liquid supply system and sensor according to anembodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 schematically depicts a lithographic apparatus according to oneembodiment of the invention. The apparatus comprises:

-   -   an illumination system (illuminator) IL configured to condition        a radiation beam PB (e.g. UV radiation or DUV radiation).    -   a support structure (e.g. a mask table) MT constructed to        support a patterning device (e.g. a mask) MA and connected to a        first positioner PM configured to accurately position the        patterning device in accordance with certain parameters;    -   a substrate table (e.g. a wafer table) WT constructed to hold a        substrate (e.g. a resist-coated wafer) W and connected to a        second positioner PW configured to accurately position the        substrate in accordance with certain parameters; and    -   a projection system (e.g. a refractive projection lens system)        PL configured to project a pattern imparted to the radiation        beam PB by patterning device MA onto a target portion C (e.g.        comprising one or more dies) of the substrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure holds the patterning device in a manner thatdepends on the orientation of the patterning device, the design of thelithographic apparatus, and other conditions, such as for examplewhether or not the patterning device is held in a vacuum environment.The support structure can use mechanical, vacuum, electrostatic or otherclamping techniques to hold the patterning device. The support structuremay be a frame or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system”.

As here depicted, the apparatus is of a transmissive type (e.g.employing a transmissive mask). Alternatively, the apparatus may be of areflective type (e.g. employing a programmable mirror array of a type asreferred to above, or employing a reflective mask).

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

Referring to FIG. 1, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus may beseparate entities, for example when the source is an excimer laser. Insuch cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus, for example when the source is a mercury lamp.The source SO and the illuminator IL, together with the beam deliverysystem BD if required, may be referred to as a radiation system.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam PB is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam PB passes through the projection system PL, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF (e.g. an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g. so as to position different target portions Cin the path of the radiation beam PB. Similarly, the first positioner PMand another position sensor (which is not explicitly depicted in FIG. 1)can be used to accurately position the mask MA with respect to the pathof the radiation beam PB, e.g. after mechanical retrieval from a masklibrary, or during a scan. In general, movement of the mask table MT maybe realized with the aid of a long-stroke module (coarse positioning)and a short-stroke module (fine positioning), which form part of thefirst positioner PM. Similarly, movement of the substrate table WT maybe realized using a long-stroke module and a short-stroke module, whichform part of the second positioner PW. In the case of a stepper (asopposed to a scanner) the mask table MT may be connected to ashort-stroke actuator only, or may be fixed. Mask MA and substrate W maybe aligned using mask alignment marks M1, M2 and substrate alignmentmarks P1, P2. Although the substrate alignment marks as illustratedoccupy dedicated target portions, they may be located in spaces betweentarget portions (these are known as scribe-lane alignment marks).Similarly, in situations in which more than one die is provided on themask MA, the mask alignment marks may be located between the dies.

The depicted apparatus could be used in at least one of the followingmodes:

-   -   1. In step mode, the mask table MT and the substrate table WT        are kept essentially stationary, while an entire pattern        imparted to the radiation beam is projected onto a target        portion C at one time (i.e. a single static exposure). The        substrate table WT is then shifted in the X and/or Y direction        so that a different target portion C can be exposed. In step        mode, the maximum size of the exposure field limits the size of        the target portion C imaged in a single static exposure.    -   2. In scan mode, the mask table MT and the substrate table WT        are scanned synchronously while a pattern imparted to the        radiation beam is projected onto a target portion C (i.e. a        single dynamic exposure). The velocity and direction of the        substrate table WT relative to the mask table MT may be        determined by the (de-)magnification and image reversal        characteristics of the projection system PL. In scan mode, the        maximum size of the exposure field limits the width (in the        non-scanning direction) of the target portion in a single        dynamic exposure, whereas the length of the scanning motion        determines the height (in the scanning direction) of the target        portion.    -   3. In another mode, the mask table MT is kept essentially        stationary holding a programmable patterning device, and the        substrate table WT is moved or scanned while a pattern imparted        to the radiation beam is projected onto a target portion C. In        this mode, generally a pulsed radiation source is employed and        the programmable patterning device is updated as required after        each movement of the substrate table WT or in between successive        radiation pulses during a scan. This mode of operation can be        readily applied to maskless lithography that utilizes        programmable patterning device, such as a programmable mirror        array of a type as referred to above.

Combinations and/or variations on the above described modes of use orentirely different modes of use may also be employed.

A further immersion lithography solution with a localized liquid supplysystem is shown in FIG. 4. Liquid is supplied by two groove inlets IN oneither side of the projection system PL and is removed by a plurality ofdiscrete outlets OUT arranged radially outwardly of the inlets IN. Theinlets IN and OUT can be arranged in a plate with a hole in its centerand through which the projection beam is projected. Liquid is suppliedby one groove inlet IN on one side of the projection system PL andremoved by a plurality of discrete outlets OUT on the other side of theprojection system PL, causing a flow of a thin film of liquid betweenthe projection system PL and the substrate W. The choice of whichcombination of inlet IN and outlets OUT to use can depend on thedirection of movement of the substrate W (the other combination of inletIN and outlets OUT being inactive).

Another immersion lithography solution with a localized liquid supplysystem solution which has been proposed is to provide the liquid supplysystem with a liquid confinement structure which extends along at leasta part of a boundary of the space between the final element of theprojection system and the substrate table. The liquid confinementstructure is substantially stationary relative to the projection systemin the XY plane though there may be some relative movement in the Zdirection (in the direction of the optical axis). A seal is formedbetween the liquid confinement structure and the surface of thesubstrate. In an embodiment, the seal is a contactless seal such as agas seal. Such a system with a gas seal is disclosed in U.S. patentapplication Ser. No. 10/705,783, hereby incorporated in its entirety byreference.

An embodiment of the invention is depicted in FIG. 5. In thisembodiment, a reservoir 10 forms a contactless seal to the substratearound the image field of the projection system so that liquid isconfined to fill a space between the substrate surface and the finalelement of the projection system. The reservoir is formed by a liquidconfinement structure 12 positioned below and surrounding the finalelement of the projection system PL. Liquid is brought into the spacebelow the projection system and within the liquid confinement structure12, e.g. via inlet 13. The liquid confinement structure 12 extends alittle above the final element of the projection system and the liquidlevel rises above the final element so that a buffer of liquid isprovided. The liquid confinement structure 12 has an inner peripherythat at the upper end closely conforms to the shape of the projectionsystem or the final element thereof and may, e.g., be round. At thebottom, the inner periphery closely conforms to the shape of the imagefield, e.g., rectangular though this need not be the case.

The liquid is confined in the reservoir by a gas seal 16 between thebottom of the liquid confinement structure 12 and the surface of thesubstrate W. The gas seal is formed by gas, e.g. air, synthetic air, N₂or an inert gas, provided under pressure via inlet 15 to the gap betweenliquid confinement structure 12 and substrate and extracted via firstoutlet 14. The overpressure on the gas inlet 15, vacuum level on thefirst outlet 14 and geometry of the gap are arranged so that there is ahigh-velocity gas flow inwards that confines the liquid.

A sensor 20 is present within the reservoir 10, such that when thereservoir is filled with liquid, sensor 20 is also exposed to theliquid. Sensor 20 has surface exposed to the liquid that comprises asemi-conductive or conductive coating covering at least a part of thesurface or is made of a semi-conductive or conductive material. In anembodiment, the sensor is an optical sensor designed to sense foralignment of the projection beam. Such a sensor may have a patternedcoating of chrome on its surface.

Power supply 21 is connected to sensor 20 in such a manner as to supplya proper bias voltage to the surface of the sensor with respect to thereservoir casing 22. The bias voltage supplied should typically beappropriate to protect the coating or material from dissolution into theliquid. Power supply 21 may be the sensor's own power supply which isconnected to the surface in order to provide the desired voltage.Alternatively, a separate power supply may be used. In the example inwhich the coating of the sensor comprises chrome and the casing 22comprises steel, a positive voltage may be applied to the casing 22 anda negative voltage to the chrome coating of sensor 20. Typically thevoltage is in the region of 5V, but voltages of up to 10V are alsoappropriate.

Application of a proper bias voltage to the sensor surface reduces orprevents formation of metal ions which dissolve in the liquid by makingsuch ion formation electrochemically unfavorable. This may result in anincreased lifetime of the sensor, and a reduced frequency ofrecalibration. The reduction of ion formation in the liquid may alsoreduce liquid contamination and hence improve other aspects of theaccuracy of the sensor or apparatus.

The sensor may be other than a chrome coated optical sensor. In thiscase, the size and polarity of the bias voltage may be varied from thosestated above in order to minimize ion formation of the specific sensorsurface in question. Further, the voltage may be applied with respect toany one or more conductive or semi-conductive parts which are exposed tothe liquid. Typically, the one or more parts are used which have a largebulk compared to the surface of the sensor. Thus, if application of abias voltage causes a small amount of corrosion of these bulky parts,their functioning is not compromised. Parts which may be used includethe reservoir casing 22 and a (steel) casing 23 of the final element ofthe projection system 24.

The bias voltage may be applied at any or at all times. In anembodiment, the voltage is applied at all times when the sensor surfaceis in contact with liquid, but not when the sensor surface is dry asthis may attract dust.

In European Patent Application No. 03257072.3, the idea of a twin ordual stage immersion lithography apparatus is disclosed. Such anapparatus is provided with two tables for supporting a substrate.Leveling measurements are carried out with a table at a first position,without immersion liquid, and exposure is carried out with a table at asecond position, where immersion liquid is present. Alternatively, theapparatus has only one table.

Although specific reference may be made in this text to the use oflithographic apparatus in the manufacture of ICs, it should beunderstood that the lithographic apparatus described herein may haveother applications, such as the manufacture of integrated opticalsystems, guidance and detection patterns for magnetic domain memories,flat-panel displays, liquid-crystal displays (LCDs), thin-film magneticheads, etc. The skilled artisan will appreciate that, in the context ofsuch alternative applications, any use of the terms “wafer” or “die”herein may be considered as synonymous with the more general terms“substrate” or “target portion”, respectively. The substrate referred toherein may be processed, before or after exposure, in for example atrack (a tool that typically applies a layer of resist to a substrateand develops the exposed resist), a metrology tool and/or an inspectiontool. Where applicable, the disclosure herein may be applied to such andother substrate processing tools. Further, the substrate may beprocessed more than once, for example in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 248, 193, 157 or 126 nm).

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, including refractiveand reflective optical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. For example, the invention may take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program stored therein.

One or more embodiments of the present invention may be applied to anyimmersion lithography apparatus, in particular, but not exclusively, tothose types mentioned above. A liquid supply system is any mechanismthat provides a liquid to a space between the projection system and thesubstrate and/or substrate table. It may comprise any combination of oneor more structures, one or more liquid inlets, one or more gas inlets,one or more gas outlets, and/or one or more liquid outlets, thecombination providing and confining the liquid to the space. In anembodiment, a surface of the space may be limited to a portion of thesubstrate and/or substrate table, a surface of the space may completelycover a surface of the substrate and/or substrate table, or the spacemay envelop the substrate and/or substrate table.

The descriptions above are intended to be illustrative, not limiting.Thus, it will be apparent to one skilled in the art that modificationsmay be made to the invention as described without departing from thescope of the claims set out below.

1. A lithographic projection apparatus, comprising: a substrate tableconstructed to hold a substrate; a projection system configured toproject a patterned radiation beam onto the substrate; a liquid supplysystem configured to at least partially fill a space between theprojection system and the substrate table with a liquid; a sensor havinga surface, the surface configured to be at least partially exposed tothe space; and an electrical power supply configured to supply a biasvoltage between the surface of the sensor and a conductive orsemi-conductive element, which element is configured to be at leastpartially exposed to the space.
 2. The apparatus according to claim 1,wherein the surface is at least partially covered with a coating.
 3. Theapparatus according to claim 2, wherein the coating comprises chrome. 4.The apparatus according to claim 3, wherein a negative voltage isapplied to the surface of the sensor and a positive voltage is appliedto the conductive or semi-conductive element.
 5. The apparatus accordingto claim 1, wherein the surface comprises a semi-conductive orconductive material.
 6. The apparatus according to claim 1, wherein theconductive or semi-conductive element comprises steel.
 7. The apparatusaccording to claim 1, wherein a negative voltage is applied to thesurface of the sensor and a positive voltage is applied to theconductive or semi-conductive element.
 8. The apparatus according toclaim 1, wherein the bias voltage is from about 5 V to about 10 V.
 9. Alithographic apparatus, comprising: an illumination system configured tocondition a radiation beam; a support constructed to hold a patterningdevice, the patterning device configured to impart the radiation beamwith a pattern in its cross-section to form a patterned radiation beam;a substrate table constructed to hold a substrate; a projection systemconfigured to project the patterned radiation beam onto a target portionof the substrate; a liquid supply system configured to at leastpartially fill a space between the projection system and the substratetable with a liquid; a sensor having a semi-conductive or conductivesurface configured to be at least partially exposed to the space; and anelectrical power supply configured to supply a bias voltage between thesurface of the sensor and an element configured to be at least partiallyexposed to the space.
 10. The apparatus according to claim 9, whereinthe surface is at least partially covered with a coating.
 11. Theapparatus according to claim 10, wherein the coating comprises chrome.12. The apparatus according to claim 11, wherein a negative voltage isapplied to the surface of the sensor and a positive voltage is appliedto the element.
 13. The apparatus according to claim 9, wherein theelement comprises a semi-conductive or conductive material.
 14. Theapparatus according to claim 13, wherein the semi-conductive orconductive material comprises steel.
 15. The apparatus according toclaim 9, wherein a negative voltage is applied to the surface of thesensor and a positive voltage is applied to the element.
 16. Theapparatus according to claim 9, wherein the bias voltage is from about 5V to about 10 V.
 17. A device manufacturing method, comprising:supplying a liquid to a space between a projection system of alithographic apparatus and a substrate; applying a bias voltage betweena surface of a sensor of the lithographic apparatus and a conductive orsemi-conductive element which element is at least partially exposed tothe liquid, the surface being at least partially exposed to the liquid;and projecting a patterned beam of radiation, using the projectionsystem, through the liquid onto the substrate.
 18. The method accordingto claim 17, wherein the surface is at least partially covered with acoating.
 19. The method according to claim 18, wherein the coatingcomprises chrome.
 20. The method according to claim 19, wherein anegative voltage is applied to the surface of the sensor and a positivevoltage is applied to the conductive or semi-conductive element.
 21. Themethod according to claim 17, wherein the surface comprises asemi-conductive or conductive material.
 22. The method according toclaim 17, wherein the conductive or semi-conductive element comprisessteel.
 23. The method according to claim 17, comprising applying anegative voltage to the surface of the sensor and applying a positivevoltage to the conductive or semi-conductive element.
 24. The methodaccording to claim 17, wherein the bias voltage is from about 5 V toabout 10 V.
 25. A method of reducing dissolution of a surface of asensor of a lithographic apparatus, the method comprising: supplying aliquid to a space between a projection system of a lithographicapparatus and a substrate; and applying a bias voltage between a surfaceof a sensor of the lithographic apparatus and a conductive orsemi-conductive element which element is at least partially exposed tothe liquid, the surface being at least partially exposed to the liquid.26. The method according to claim 25, wherein the surface is at leastpartially covered with a coating.
 27. The method according to claim 26,wherein the coating comprises chrome.
 28. The method according to claim27, wherein a negative voltage is applied to the surface of the sensorand a positive voltage is applied to the conductive or semi-conductiveelement.
 29. The method according to claim 25, wherein the surfacecomprises a semi-conductive or conductive material.
 30. The methodaccording to claim 25, wherein the conductive or semi-conductive elementcomprises steel.
 31. The method according to claim 25, comprisingapplying a negative voltage to the surface of the sensor and applying apositive voltage to the conductive or semi-conductive element.
 32. Themethod according to claim 25, wherein the bias voltage is from about 5 Vto about 10 V.